The present invention relates to an image sensor array, such as found, for example, in digital scanners, copiers, and facsimile machines. More particularly, the present invention relates to a selectable amplifier which can be associated with at least one individual photosensor in such a sensor array.
Image sensor arrays typically comprise a linear array of photosensors which scan an image-bearing document and convert the small image areas viewed by each photosensor to image signal charges. Following an integration time, the image signal charges are amplified and transferred to a common output line or bus through successively actuated multiplexing transistors.
In the scanning process, bias and reset charges are applied to each photosensor (such as a photodiode) in a predetermined time sequence during each scan cycle. In a particular embodiment of such an image sensor array, a two-stage transfer circuit is provided for transferring the image signal charges from the photodiodes. A bias charge is applied to each photodiode through a bias charge injection transistor coupled to a node between the photodiode and the input to the transfer circuit. From the transfer circuit, the image-based charges are caused to pass through an amplifier circuit, one amplifier circuit being typically provided for each photodiode, or at least to each RGB-color triplet of color-sensitive photodiodes.
In an active photosensor array it is typical to have individual photosensors transfer image-related charges therefrom onto capacitors, one capacitor being associated with each photosensor. This xe2x80x9creset capacitorxe2x80x9d temporarily stores the image-related charge, which is subsequently transferred through an amplifier associated with the reset capacitor. Then the amplified voltage signal resulting from the transfer is read out as a video signal on a video line.
The present invention is generally directed to the design of an amplifier which can be associated with an individual reset capacitor which in turn is associated with a particular photosensor in an image sensor array. As is known in the prior art, particularly in the ""423 patent referenced below, a preferred design for an image sensor array having an amplifier associated on a one-to-one basis with a set of photosensors is to have each amplifier be xe2x80x9cselectablexe2x80x9d so that the amplifier is fully operational only during the short period of time when the particular amplifier is transferring a charge from a reset capacitor onto the video line: for the bulk of the time of operation, when the particular reset capacitor is not transferring a signal onto the video line, the amplifier is xe2x80x9cdeselected,xe2x80x9d or another words put in a low-power-consumption mode, thus saving power consumption of the entire array.
A practical problem which occurs when using selectable amplifiers in the image sensor array is that, when an amplifier associated with a particular reset capacitor and photosensor is deselected following a signal transfer, a residual charge remains within the amplifier circuitry, and this residual charge will capacitively couple unpredictable amounts of charge onto the reset node capacitance for the next time an image-related charge is transferred from the capacitor onto the video line. This unpredictable additional amount of charge thus placed on the reset capacitor will distort the signal charge of the subsequent image reading.
It is an object of the present invention to provide circuitry for a selectable pixel amplifier which will reset to a known voltage following every readout from a reset capacitor onto a video line.
Bazes, xe2x80x9cTwo Novel Fully Complementary Self-Biased CMOS Differential Amplifiers,xe2x80x9d IEEE Journal of Solid-State Circuits, Vol. 26, No. 2, February 1991, pp. 165-168, discloses designs of differential amplifiers having fully complementary configurations and which are self-biased through negative feedback. U.S. Pat. Nos. 4,857,476 and 4,958,133 by the same author show related amplifier designs.
U.S. Pat. No. 5,493,423 discloses an amplifier circuit which can be associated with an individual photosensor in an image sensor array. With each cycle of passing an image signal through an amplifier, a low standby current is applied to certain transistors within the amplifier until the next signal is to be output. Critical nodes within the amplifier are caused to settle to known charge values before each image signal is passed therethrough.
According to one aspect of the present invention, there is provided an image sensor array, comprising a plurality of photosensors, with a transfer circuit associated with each photosensor. Selection means cause each transfer circuit to output an image-related charge to an output line. An amplifier is interposed between each transfer circuit and the output line, the amplifier including an input node connected to the reset capacitor, and an input stage connected to the input node. The input stage includes a plurality of transistors, the transistors in the input stage forming a set of input stage nodes capacitively coupled to the input node. Reset means cause all of the input stage nodes in the amplifier to be biased to at least one known magnitude incidental to the amplifier transferring a charge from the transfer circuit to the output line.